1. Field of the Invention
The present invention relates generally to a process for producing a ceramic arc tube for use in a high-pressure metal-vapor discharge lamp, and more particularly to a process suitable for producing a ceramic arc tube for such a discharge lamp with high dimensional accuracy, which arc tube includes a translucent ceramic tube, wherein at least one of opposite ends of the ceramic tube is closed by a ceramic end cap that supports a discharge electrode.
2. Discussion of the Prior Art and Problem Solved by the Invention
In a known high intensity discharge lamp (hereinafter referred to as "HID lamp") such as sodium lamps and metal-halide lamps, a translucent arc tube (which forms a body of an arc tube of the lamp) is formed from a ceramic tube having high corrosion resistance and high light transmissivity, in particular, a translucent alumina tube. The opposite ends of such a translucent ceramic arc tube are closed by ceramic end caps which support respective discharge electrodes made of tungsten or molybdenum.
There is known a process for fabricating such an enclosed ceramic arc tube having a translucent ceramic tube closed at their opposite ends by respective ceramic end caps as indicated above. An example of such a process is disclosed in Toku-Ko-Sho No. 50-30384 (Publication for opposition purpose in 1975) of Japanese Patent Application. According to the process disclosed therein, end plate blanks giving the ceramic end caps and a tubular blank giving the ceramic arc tube are subjected to respective preliminary firing operations at comparatively low temperatures, in order to burn out organic substances included in the blanks. Then, the fired end plate blanks are fitted into the opposite ends of the fired tubular blank, and a thus obtained assembly of the end plate blanks and the tubular blank is subjected to a secondary firing operation at a relatively high temperature in the neighborhood of 1900.degree. C. As a result of the secondary firing operation, the tubular blank is made translucent, and the fired end plate blanks are secured integrally to the tubular blank due to a difference in the shrinkage between the end plate and tubular blanks during the secondary firing.
In the conventional fabrication process described above, the fitting of the end plate blanks into the tubular blank is conducted after these blanks have been hardened by the preliminary firing. Hence, gaps may arise at the interface of the end plate blanks and the tubular blank after the assembly is subjected to the secondary firing. Thus, the ceramic arc tube fabricated by the conventional process may suffer form leakage of a gas through such gaps.
The end plate blanks conventionally used for the end caps usually take the form of a ring-like disc adapted to engage the open ends of the tubular blank for the ceramic arc tube. These disc blanks are difficult to be positioned relative to the tubular blank with consistent accuracy. Further, the disc blanks fitted in the tubular blank may be dislocated or displaced relative to the tubular blank, during transportion or handling of the assembly of the disc and tubular blanks, or due to the vibrations, or impacts thereto during transportation of the assembled blanks to and from a firing furnace. Such dislocation or displacement may lower the dimensional accuracy of the obtained ceramic arc tube, i.e., may result in a large variation or fluctuation in the distance between the discharge electrodes supported by the end caps, causing a problem of reduced discharging stability of the metals filling the arc tube. To solve this problem, the conventional process employs grinding of the end portions of the fired translucent ceramic arc tube, for reducing the possible dimensional variations to within predetermined tolerances. However, such a grinding step is cumbersome, and is not effective to correct a variation in the distance between the inner surfaces of the end caps.